In the oil-free screw compressor, air is compressed by a pair of male and female screw rotors which is rotatable in an oil no-supply state and in a non-contact state. In the oil-free screw compressor, there may be a case where compressed air produced in a rotor chamber is leaked along a rotary shaft or a lubricant supplied to a gear which drives the rotary shaft or a bearing which supports the rotary shaft flows into the rotor chamber. To prevent such drawbacks, a shaft seal device is disposed between the rotor chamber and the bearing. The shaft seal device includes: an air seal portion which seals compressed air from the rotor chamber; and an oil seal portion which seals a lubricant from the bearing.
When the rotor chamber is brought into a negative pressure state during an unloading operation, there may be a case where a lubricant which is supplied to the bearing or the like flows into the inside of the rotor chamber after passing through the oil seal portion although an amount of the lubricant is insignificant. To prevent such flowing in of the lubricant into the inside of the rotor chamber, an atmosphere open passage is provided for making an air ventilation gap formed on a rotor-chamber-side end portion of the oil seal portion and an atmosphere side of a casing communicate with each other. When the rotor chamber is brought into a negative pressure state, atmospheric air is introduced into the ventilation gap through the atmosphere open passage thus preventing the lubricant from flowing into the rotor chamber.
The oil-free screw compressor provided with the above-mentioned shaft seal device is disclosed in JP 2011-256828 A and JP 2008-255796 A, for example.